Radiant heat reflection in devices such as getter pumps

ABSTRACT

A heat reflector having a number of discrete spaced surfaces which present a substantially continual heat-reflecting surface wherein adjacent surfaces are offset with respect to each other sufficiently to provide for the passage of gas molecules at pressures within the molecular flow region as well as at higher pressures. These reflectors which have a high conductance to gases are especially useful when employed with getter pumps wherein they minimize radiant heat losses and reduce the amount of current necessary to heat the getter material employed in these pumps.

United States Patent Inventors Mario Zucchinelli;

Bruno Ferrurio, both of Milan, Italy Appl. No. 866,335 Filed Oct. I4,I969 Patented Sept. 7, I971 Assignee S. A. E. S. Getters S.p.A. Milan,Italy Priority Oct. 28, 1968 Italy 23038 A/68 RADIANT HEAT REFLECTION INDEVICES SUCH AS GETTER PUMPS I3 Claims, 6 Drawing Figs.

US. Cl 417/51, 3 1 3/7 Int. Cl F04f 11/00, H01 j 7/16 Field of Search417/48, 49,

[56] References Cited UNITED STATES PATENTS 2,482,043 9/1949 Veehemanset al 41 7/48 3,214,245 l0/l965 Peters, Jr 4l7/49 X PrimaryExaminer-Robert M. Walker Attorney-David R. M urphy PATENTEDSEP Tran3503704 RADIANT HEAT REFLECTION IN DEVICES SUCK-ll AS GET'IER PUMPSGetter pumps employing a substrate coated with a head activablenonevaporable getter material are widely employed to produce andmaintain vacuum in dynamic and static systems having pressures down toand including the molecular flow range of pressure. In some systems thegetter pump can be placed in a separate envelope which is in fluidcommunication with the vacuum chamber of the system. In such casesessentially only the envelope is heated while supplying current to thepump. However in other systems the getter pump is generally mounted inthe system itself causing two problems. First additional electric powermust be supplied to the pump to maintain the getter material at thedesired temperature while compensating for heat loss by radiation.Second the radiated heat increases the temperature of parts of thevacuum system causing undesirable gas evolution which reduces the degreeof vacuum and can cause other problems. These getter pumps generallycomprise a central resistance heater coaxially surrounded by the coatedsubstrate which is frequently pleated. In operation current is passedthrough the central resistance heater radiating heat to the gettermaterial heating and activating it. While such getter pumps have foundwide acceptance they generally require a large amount of powerprincipally due to the loss of heat from the getter pump by radiationespecially when the pump is mounted in its nude version. Providing thesepumps i.e. the nude ones, with commonly employed thermal insulation isimpractical for a number of reasons. First many commonly employedthermal insulators are formed of fibrous particulate materials and arenot compatible with advanced vacuum techniques. These insulators areprone to flake off causing problems within the chamber to be evacuated.Second most common insulation materials cannot withstand the hightemperatures employed to degas these materials. Other insulationmaterials which have a low conductance to gas cannot be employed becausethey substantially reduce the pumping speed of the getter pump.Providing gas passages through such insulation materials generally alsoprovides a path for the escape of radiant heat. The usual thermalreflectors employed in the vacuum art are formed by metal sheetscomposed of a number of strata where conductance to gas and volumeoccupied do not present problems. Radiant heat reflectors commonlyemployed for other purposes have been found to be unsuitable for usewith getter pumps for the above and other reasons.

It is therefore an object of the present invention to provide a radiantheat reflector which is substantially free of one or more of thedisadvantages of prior reflectors. Another object is to provide a novelcombination of a getter pump and a radiant heat reflector.

A further object is to provide a heat reflector having a highconductance to gas at pressures within the molecular flow re gion aswell as at atmospheric pressures.

A still further object is to provide a heat reflector constructedentirely of materials resistant to the temperatures commonly employedfor degassing.

Additional objects and advantages of the present invention will beapparent by reference to the following detailed description and drawingswherein:

FIG. II is an exploded view of the getter pump of the present inventionwith a radiant heat reflector wherein the reflector is shown in FIG. laand the remainder of the getter pump is shown in FIG. lb;

FIG. 2 is a top view of the reflector of FIG. la;

FIG. 3 is a sectional view taken along line 3-3 of FIG. la;

FIG. 4 is a sectional view taken along line ll-43 of FIG. 2;

FIG. 5 is a sectional view taken along line 5-5 of FIG. 3. According tothe present invention the above and other objects are accomplished byproviding a novel heat reflector comprising a plurality of discretespaced surfaces, the surfaces presenting a substantially continualheat-reflecting surface, adjacent surfaces being offset with respect toeach other sufficiently to provide for the passage of gas molecules atpressures within the molecular flow region as well as at higherpressures. Referring now to the drawings and in particular to FIG. la

there is shown in the form of a reflector 10 a nonlimiting embodiment ofthe present invention. The reflector l0 comprises an upper support Illand a lower support 12. A plurality of straight strips 13 and offsetstrips 14 are attached at their ends to the upper support ill and thelower support l2.

As shown in FIGS. 2 and 4, the upper support ll is provided with aplurality of elongated passages l5, large holes l6, and small holes l7whose function is to further increase the gas conductance of thereflector 10. The upper support ll also has an axially positioned hole118 by which the reflector R0 is attached to the remainder of the getterpump as described below. As shown in FIG. 4 the upper support Ill isprovided with an annular groove 19 adapted to receive the straightstrips 113 and the offset strips 114. The upper support ll is alsoprovided with an annular ring 20 adapted to position the upper end ofthe reflector on the remainder of the getter pump.

As shown in FIG. 5, the lower support 12 is likewise provided with anannular groove 21 adapted to contain the straight strips l3 and theoffset strips 14. The lower support 12 has an inner surface 22 adaptedto position the lower end of the reflector 10 on the remainder of thegetter pump. The lower support 12 can be described as torroidal. Asshown in FIGS. 4 and 5, the straight strips 13 are fixedly attached atone end in the annular groove 19 of the upper support 11 and at theother end in the annular groove 21 of the lower support H2. The offsetstrips 14 have a relatively long straight segment 23 attached to anupper inwardly angled segment 24 and a lower inwardly angled segment 25which in turn are attached to an upper terminal segment 26 and a lowerterminal segment 27. The terminal segments 26 and 27 as well as the endsof the straight strips 13 are fixedly held in the grooves 19 and M byany convenient means such as spot welding diagrammatically shown in FIG.la as small indentions 28.

Referring now to FIG. llb there is shown a getter cartridge 30comprising an upper retainer 31 and a lower retainer 32, both ofgenerally cylindrical shape, having a screen 33 also of cylindricalshape fixedly therebetween. Axially disposed within the cartridge 30 isa rod 34 the upper end of which is threaded. Surrounding the rod 34 andwithin the cartridge 30 is an insulator 35 having wound thereon a wire36 of high electrical resistance which can be connected to a source ofpower not shown in order to provide heat for the cartridge 30. Withinthe cartridge 30 and coaxially disposed around the rod 34 are a seriesof pleated strips 37 in stacked array coaxially held by means of thescreen 33. Attached to the central portion of the strips 37 is aparticulate nonevaporable getter material 38 which is heat activatable.The strips 37 are folded back and forth in a pleated manner in order toprovide a very high total surface area of getter material 38 for gassorption.

The getter pump of the present invention comprising the reflector l0 andthe cartridge 30 is placed in operation by moving the reflector 10downwardly until the rod 34 of the cartridge 30 passes through the hole118 of the upper support lll whereupon the reflector E0 is fixedly heldto the cartridge 30 by means of a nut not shown. The annular ring 20fits snugly against the outside of the upper retainer 31 and the sur'face 22 of the lower support 12 fits snugly against the outside of thelower retainer 32. The wire 36 is heated by connecting it to any sourceof electrical power. The heat is radiated from the wire 36 to the strips37 heating and activating the getter material 38, rendering it gassorptive and creating a very low pressure within the cartridge 30. Byvirtue of this low pressure gases pass between the strips l3 and M andare sorbed by the getter material 38. When the getter pump is operatingin the molecular flow region wherein the path of gas molecules isstraight, the gas molecules can reach the getter material 38 by a pathbetween the strips l3 and 14 such as shown by arrow 39.

As shown in FIG. 3 the length, w,, of each straight strip 13 issubstantially equal to the width, w, of each offset strip 14 whereas thesum of the widths of all the straight strips 13 and all the offsetstrips 14 is substantially equal to the circumference of each annulargroove 19 or 21. By virtue of this particular geometric arrangement andsince the offset strips 14 have inwardly extending segments 24 and 25,the central segment 23 of the offset strip 14 is a chord of a circlehaving a radius smaller than the radius of the annular grooves 19 and21. Since heat is normally radially radiated from the cartridge 30 asshown by arrows 40 and 41 that radiated heat which just misses the innersurface of the offset strip 14 must impinge and be reflected by theinner surface of the straight strip 13. Since these inner surfaces arerendered highly heat reflective by any convenient means such aspolishing, the amount of heat produced by the wire 36 which escapes fromthe getter pump is greatly reduced. Since the annular ring 20 of theupper support 11 and the inner surface 22 of the lower support 12 holdthe reflector l axially on the cartridge 30, points of contact betweenthe reflector and the cartridge 30 are limited to a small portion of theupper reflector support 11 and the upper cartridge retainer 30 as wellas the lower reflector support 12 and the lower cartridge retainer 32.Since the portions of the offset strip 14 are maintained out of contactwith the cartridge further minimizing the heat loss.

All parts of the reflector 10 including the supports 11 and 12 and thestrips 13 and 14 as well as the parts of the cartridge 30 are preferablyconstructed of materials which are both temperature and vacuumcompatible and preferably are constructed of metals which can withstandthe elevated temperatures such as 350 C. to 800 C. commonly employed indegassing. Examples of suitable metals include iron and stainless steel.

As used herein the molecular flow region refers to that pressure rangewherein gas flows under conditions such that the largest internaldimension of a transverse section of the vessel is smaller than the meanfree path of the molecules of the gas. In the molecular flow region therate of flow of gas is limited not by collisions between molecules butby collisions of molecules with the walls of the vessel. For vesselscommonly employed as vacuum tubes having a largest internal dimension ofabout 1 to 50 cm. the molecular flow region is generally from just abovezero up to about 10 torr. As used herein the conductance (F) of thereflector 10 in the molecular flow region for a given gas is the ratioof throughput of gas (Q) to the partial pressure difference across thereflector 10 (P P,) in the steady state. it is measured in liters persecond, and given by F=Q/(P P,); where P is the upstream pressure, and Pis the downstream pressure. 7

Although the invention has been described in considerable detail withreference to certain preferred embodiments thereof, it will beunderstood that variations and modifications can be effected within thespirit and scope of the invention as described above and as defined inthe appended claims.

What is claimed is:

1. A heat reflector having a high conductance to gases comprising aplurality of discrete spaced surfaces, the surfaces gases comprising aplurality of strips having a heat reflective inner surface said stripsbeing generally circularly disposed on at least two circles of differentradii; adjacent strips being on different circles; the inner surfaces ofall strips being substantially lperpendicular to the radii.

4. he reflector of claim 3 wherein the ends of all strips are aligned onthe circle oflargest radius.

5. The reflector of claim 4 wherein the sum of the widths of the stripsis equal to the circumference of the circle of the largest radius.

6. A cylindrical, radiant-heat reflector having a high conductance togases comprising a plurality of planar strips having a heat reflectiveinner surface, said strips being circularly disposed as chords on twocircles of different radii; alternate strips being on alternate circles;the heat-reflective surfaces of each strip being perpendicular to theradii.

7. A radiant heat reflector having a high conductance to gasescomprising:

A. a first support;

B. a second support;

C. a series of straight strips fixedly attached at one end to the firstsupport and at the other end to the second sup- P D. a series of offsetstrips fixedly attached at one end to the first support and at the otherend to the second support; the offset strips being disposed betweenadjacent straight strips, whereby the straight strips and the offsetstrips present a continual heat-reflecting surface while permitting thepassage ofgas therebetween. 8. A cylindrical, radiant heat reflectorofclaim 7 wherein adjacent straight strips are spaced from one anotherand have an inner heat reflective surface;

wherein the sides of the ends of the offset strips contact adjacentstraight strips; each strip comprising two terminal segments eachattached to two inwardly angled segments attached to a straight centralsegment leaving an inner heat reflective surface.

9. A getter pump comprising a substrate coated with a nonevaporablegetter material, means for heating the nonevaporable getter material andmeans for reflecting radiant heat.

10. A getter pump of claim 9 wherein the means for reflecting radiantheat comprises a heat reflector having a high conductance to gasescomprising a plurality of discrete spaced presenting a substantiallycontinual heat-reflecting surface,

surfaces, the surfaces presenting a substantially continual heatreflecting surface, adjacent edges of adjacent surfaces being offsetwith respect to one another sufficiently to allow the passage of gasesin a straight path between adjacent surfaces.

11. A getter pump comprising a circular pleated substrate coated with anonevaporable getter material, means for heating the nonevaporablegetter material and means for reflecting radiated heat said heatreflecting means substantially surrounding the circular pleatedsubstrate.

12 A getter pump comprising a substrate coated with a nonevaporablegetter material, means for heating the nonevaporable getter material anda heat reflector having a high conductance to gases comprising aplurality of discrete spaced surfaces, the surfaces presenting asubstantially continual heat-reflecting surface, adjacent surfaces beingoffset with respect to each other sufficiently to allow the passage ofgas.

13. A getter pump comprising a substrate coated with a nonevaporablegetter material, means for heating the nonevaporable getter material anda radiant heat reflector having a high conductance to gases comprising aplurality of strips having a heat reflective inner surface said stripsbeing generally circularly disposed on at least two circles of differentradii; adjacent strips being on different circles, the inner surfaces ofall strips being substantially perpendicular to the radii.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 603704 D8td Sgpt 7 I 1 971 Inventor(s) Mario Zucchinelli. Bruno Eerrar'ioIt is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Col. 1, line l delete "head", insert heat Col. 3, 'line 46 delete "10insert "10' Signed and sealed this I 8th day of January 1 972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR ROBERT GOTTSCHALK Attesting Officer ActingCommissioner of Patents DRM USCOMM-DC some-Pe a U 5 GOVERNMENT PRINTINGCFFICE 969 0-366-134

1. A heat reflector having a high conductance to gases comprising aplurality of discrete spaced surfaces, the surfaces presenting asubstantially continual heat-reflecting surface, adjacent surfaces beingoffset with respect to each other sufficiently to allow the passage ofgas.
 2. A heat reflector having a high conductance to gases comprising aplurality of discrete spaced surfaces, the surfaces presenting asubstantially continual heat-reflecting surface, adjacent edges ofadjacent surfaces being offset with respect to one another sufficientlyto allow the passage of gas in a straight path between adjacentsurfaces.
 3. A radiant heat reflector having a high conductance to gasescomprising a plurality of strips having a heat reflective inner surfacesaid strips being generally circularly disposed on at least two circlesof different radii; adjacent strips being on different circles; theinner surfaces of all strips being substantially perpendicular to theradii.
 4. The reflector of claim 3 wherein the ends of all strips arealigned on the circle of largest radius.
 5. The reflector of claim 4wherein the sum of the widths of the strips is equal to thecircumference of the circle of the largest radius.
 6. A cylindrical,radiant-heat reflector having a high conductance to gases comprising aplurality of planar strips having a heat reflective inner surface, saidstrips being circularly disposed as chords on two circles of differentradii; alternate strips being on alternate circles; the heat-reflectivesurfaces of each strip being perpendicular to the radii.
 7. A radiantheat reflector having a high conductance to gases comprising: A. a firstsupport; B. a second support; C. a series of straight strips fixedlyattached at one end to the first support and at the other end to thesecond support; D. a series of offset strips fixedly attached at one endto the first support and at the other end to the second support; theoffset strips being disposed between adjacent straight strips, wherebythe straight strips and the offset strips present a continualheat-reflecting surface while permitting the passage of gastherebetween.
 8. A cylindrical, radiant heat reflector of claim 7wherein adjacent straight strips are spaced from one another and have aninner heat reflective surface; wherein the sides of the ends of theoffset strips contact adjacent straight strips; each strip comprisingtwo terminal segments each attached to two inwardly angled segmentsattached to a straight central segment leaving an inner heat reflectivesurface.
 9. A getter pump comprising a substrate coated with anonevaporable getter material, means for heating the nonevaporablegetter material and means for reflecting radiant heat.
 10. A getter pumpof claim 9 wherein the means for reflecting radiant heat comprises aheat reflector having a high conductance to gases comprising a pluralityof discretE spaced surfaces, the surfaces presenting a substantiallycontinual heat reflecting surface, adjacent edges of adjacent surfacesbeing offset with respect to one another sufficiently to allow thepassage of gases in a straight path between adjacent surfaces.
 11. Agetter pump comprising a circular pleated substrate coated with anonevaporable getter material, means for heating the nonevaporablegetter material and means for reflecting radiated heat said heatreflecting means substantially surrounding the circular pleatedsubstrate.
 12. A getter pump comprising a substrate coated with anonevaporable getter material, means for heating the nonevaporablegetter material and a heat reflector having a high conductance to gasescomprising a plurality of discrete spaced surfaces, the surfacespresenting a substantially continual heat-reflecting surface, adjacentsurfaces being offset with respect to each other sufficiently to allowthe passage of gas.
 13. A getter pump comprising a substrate coated witha nonevaporable getter material, means for heating the nonevaporablegetter material and a radiant heat reflector having a high conductanceto gases comprising a plurality of strips having a heat reflective innersurface said strips being generally circularly disposed on at least twocircles of different radii; adjacent strips being on different circles,the inner surfaces of all strips being substantially perpendicular tothe radii.